It is well known that tire wear is a function of the frictional work between tile tire and a supporting surface when the two are in frictional contact with each other. It is further known that tire wear at a particular location is proportional to such frictional work according to the formula W=uNs, where W=frictional work, u=coefficicnt friction between the tire and the support surface, N=normal force between the tire and support surface at the location of interest, and s=slip between tile tire and support surface at the location of interest. Accordingly, tire wear at a given location is a direct function of the product of N (normal force) and s (slip) at that location.
With a knowledge of the foregoing, one can analytically determine the propensity of a tire to wear by measuring N and s. Known prior systems have measured N and sought to calculate s, but have been generally incapable of simultaneously measuring or determining both at a given point on a tire. In general, the prior art has employed mechanical force transducers for determining the force upon the tire at an area of interface with a support surface, but such techniques and apparatus have not been given to a simultaneous determination of slip at the interface. Using such techniques and apparatus, it has been found that the slip of interest removed the area of concern from contact with the pressure transducer such that the force and slip measurements were taken from two distinctly different areas. Accordingly, the prior art has been given not only to inaccuracies in measurement, but an inability to directly correlate a determined normal force with a determined slip function. Consequently, the product Ns, intended to be proportional to frictional work and propensity for wear, is generally inaccurate and incapable of providing a reliable assessment.
There is a need in the art for a noncontact measuring or monitoring system to simultaneously and accurately obtain both slip (s) and force (N) measurements at specified locations on a tire, which measurements may then be employed to determine the propensity of wear of the tire at that specific location. There is a most important need in the art for the ability to take such simultaneous measurements to assure that the data employed to determine wear propensity are taken from the same area of the tire under the same conditions.
It has been known in the art that tire contact stresses may be employed as indicators of tread wear. Such general recognition has been pronounced in "Contact Stresses As Predictors Of Tread Wear," by W. K. Shepherd, copyright 1986 by Michelin Americas Research & Development Corp. and in "Mechanics of Pneumatic Tires," edited by Samuel K. Clark, and published by the U.S. Department of Transportation in 1981. Additionally, it has been known that force measurements may be taken from the light reflected from an illuminated plate of glass when an article is brought into forceful contacting engagement with a reflective shield received upon the plate of glass. This concept of measuring force or pressure upon a surface by the measurement of internal reflection light (IRL) is generally known and is taught in U.S. Pat. No. 4,858,621. However, the prior art has failed to teach an apparatus or technique for employing IRL as a means for determining a propensity for tread wear. More particularly, the prier art has failed to teach any such apparatus or technique whereby force and slip measurements of a tire and support surface interface could be simultaneously measured and employed for determining a propensity for tread wear.